1. Technical Field
This invention relates in general to robotics and, more particularly, to a unique mechanical robotic arm positioning system which produces a closed loop reference frame with an associated workpiece fixture or other robotic system, thereby decoupling operation of the end effector from gross movement inaccuracies or vibrations of the robotic arm and allowing less precise robots to perform high precision tasks.
2. Description of the Prior Art
Due to its cost effectiveness, robotics will continue to be increasingly important to successful competition within many industries. An industrial robot is broadly defined as a reprogrammable, multifunctional manipulator designed to move material, parts, tools, and specialized devices through variable programmed motions for performance of a variety of tasks. Commonly performed tasks include drilling, welding, spray painting, and "pick-and-place" operations. While the majority of present industrial robotic systems are acceptable for such operations, these "less precise" systems experience serious problems in applications for which a high degree of positioning precision is essential, such as with many assembly and inspection tasks. For example, very stringent tolerances are usually involved in the assembly of small electronic parts. Further, limited space between parts or components often presents additional restrictions, such as is the case with the assembly of printed circuit boards.
A typical industrial robotic system comprises a number of interconnected arm segments at the end of which is located an end effector, e.g., a gripper or micromanipulator. Gear backlash, bending deflections and calibration errors are some of the reasons for positional and rotational errors unavoidably created with respect to a target three-dimensional and angular position. The errors accumlate from the proximal arm member to the distal functional end effector. A thorough discussion of the robotic accuracy issue is provided by Chia P. Day in an article entitled: "Robot Accuracy Issues and Methods of Improvement", published by Robotics Today, Vol. 1, No. 1, Spring, 1988 edition.
Prior art devices have almost exclusively centered on correcting such errors or deviations of movement through complicated and expensive sensor and feedback means as typified by recently developed laser tracking techniques. Although such tracking systems do provide a more accurate determination of end effector position, they tend to be more complex and costly than feasible for most industrial robotic systems, and often demand precise coordinates for the entire robot path rather than simply a target location and orientation. Further, existing devices typically require that an end effector be replaced in exactly the same position if removed for maintenance or replacement, or else all previously programmed points will be offset.
Another prior art device is disclosed by Inoue in U.S. Pat. No. 4,562,391, entitled "Robotic Positioning System". Briefly, the Inoue patent describes a positioning system for an electrical discharge machining (EDM) center which includes a gyroscope assembly. The gyroscope provides directional reference for monitoring the accuracy of a spatial orientation assumed by any of the innerconnected arm members, the hand member or tool electrode at the distal end of the system, once a predetermined movement is achieved. As with the laser tracking system, however, this approach is more complex and costly than feasible for most industrial applications
The Inoue patent also describes a fail-safe for the discussed EDM system which includes a first positioning plate having the workpiece securely mounted thereon with a plurality of pins projecting from the plate. A second positioning plate is detachably mounted on the electrode holder and has a plurality of holes bored therein. The pins and holes are arranged so that when the electrode surface of the tool electrode is properly positioned with respect to the workpiece, the holes are capable of fully accepting the pins, thereby providing a check on whether the electrode at the distal end of the robotic system has been properly positioned relative to the workpiece by the gyro assembly.
As noted, the Inoue gyro assembly is believed more complex and costly than acceptable for most industrial applications. Further, the described fail-safe enhancement is of no value to proper positioning of the tool electrode. The gyro assembly maintains the reference frame on the electrode holder positioning plate parallel to the workpiece positioning plate at all times. There is no force feedback or sensoring information on whether the electrode holder positioning plate has come into contact with the pins of the workpiece positioning plate. The documented process would tend to ream out the bored holes if the robotic system is not truly already accurate, i.e., sufficiently accurate for the pins to be inserted into the corresponding holes without wedging or jamming problems arising.
Therefore, a novel positioning system which is less complex and costly than presently available robotic positioning systems and which allows high precision robotic assembly by "less precise" industrial robots is believed clearly desirable.